Light emitting devices with inhomogeneous quantum well active regions

ABSTRACT

A method of fabricating a light emitting device includes modulating a crystal growth parameter to grow a quantum well layer that is inhomogeneous and that has a non-random composition fluctuation across the quantum well layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.11/957,247, entitled LIGHT EMITTING DEVICES WITH INHOMOGENEOUS QUANTUMWELL ACTIVE REGIONS, filed Dec. 14, 2007, the disclosure of which isherein incorporated by reference in its entirety.

BACKGROUND

Semiconductor light-emitting devices such as Light-Emitting Diodes(LEDs) and laser diodes typically utilize an active layer, whereelectrons and holes combine to emit light. Table 1, which is presentedbelow, describes the epitaxial layer structure of an LED that emitslight at a wavelength of 345 nm. The structure utilizes multiple quantumwell active layers with homogeneous composition. In Table 1, the layersare numbered in order from 1 to 426, with 1 being the top most layer and426 the layer closest to the substrate. In order to save space, insituations where a specific sequence of layers is merely repeated anumber of times, the number of times that the original sequence repeatsis indicated in the “repetition” column of Table 1. For example,although layers 4-43 are not specified, the sequence of layers 2 and 3is repeated 20 times. Thus, all even layers between 2 and 44 (layers 4,6, 8, 10, . . . , 38, 40, 42) are the same as layer 2, while all oddlayers between 3 and 45 (5, 7, 9, . . . , 39, 41, 43) are the same aslayer 3.

The “composition” column of Table 1 is self-explanatory, as is the“thickness” column. Some layers are indicated as having a dopant, forexample, layer 2 (Al_(0.30)Ga_(0.70)N:Si) is doped with silicon. The“comments” column of Table 1 provides additional descriptive informationabout the layer or group of layers. For example, layers 49 and 53 arethe quantum well layers and have a relatively low band gap, while layers46-48, 50-52, and 54-56 serve as the barriers and have a relatively highband gap compared to the quantum well layers.

TABLE 1 Thickness Layer Composition (nm) Repetition Comments 1 GaN: Mg20 p-contact 2 Al_(.30)Ga_(.70)N: Si 3.06 20 p-cladding, 144.48 nm 3Al_(.28)Ga_(.72)N: Si 3.82 total 44 Al_(.26)Ga_(.74)N: Mg 66.96p-waveguide 45 Al_(.48)Ga_(.52)N: Mg 21.88 tunnel barrier layer 46In_(x)Al_((.18−x))Ga_(.82)N 4.06 barrier 47 In_(x)Al_((.18−x))Ga_(.82)N:Si 2.32 48 In_(x)Al_((.18−x))Ga_(.82)N 4.06 49In_(x)Al_((.15−x))Ga_(.85)N 5.25 quantum well 50In_(x)Al_((.18−x))Ga_(.82)N 4.06 barrier 51 In_(x)Al_((.18−x))Ga_(.82)N:Si 2.32 52 In_(x)Al_((.18−x))Ga_(.82)N 4.06 53In_(x)Al_((.15−x))Ga_(.85)N 5.25 quantum well 54In_(x)Al_((.18−x))Ga_(.82)N 4.06 barrier 55 In_(x)Al_((.18−x))Ga_(.82)N:Si 2.32 56 In_(x)Al_((.18−x))Ga_(.82)N 4.06 57In_(x)Al_((.26−x))Ga_(.74)N: Si 70.56 n-waveguide 58 Al_(.28)Ga_(.72)N:Si 3.16 100 n-cladding, 555.22 nm 59 Al_(.30)Ga_(.70)N: Si 2.33 total260 Al_(.30)Ga_(.70)N: Si 0.728 261 Al_(.31)Ga_(.69)N: Si 1198.35n-contact layer 262 Al_(.31)Ga_(.69)N 159.78 interface layer to low Al263 GaN 0.25 40 strain relief layer 264 AlN 0.38 345 GaN 0.25 39 346 AlN1.0 425 AlN 28.8 surface conditioning layer 426 Al_(.83)Ga_(.27)N 1200AlGaN template Sapphire substrate

The active region of the 345 nm LED described above in Table 1 arelayers 46-56. These layers include three barrier layers and two 5.25nm-thick quantum wells composed of In_(x)Al_((0.15-x))Ga_(0.85)N withx—the indium component—nominally at 1%.

Table 2, which is presented below, summarizes the Metal Organic ChemicalVapor Deposition (MOCVD) gas flow conditions for each of the quantumwells (layers 49 and 53) of Table 1. A constant flow rate of 0.9 cc/minof tri-methyl gallium (TMG), 0.4 cc/min of tri-methyl aluminum (TMA),and 80 cc/min of tri-methyl indium (TMI) is employed during the 120second growth time of each quantum well.

TABLE 2 Time (sec) TMG (cc/min) TMA (cc/min) TMI (cc/min) 0-120 0.9 0.480

Most light-emitting semiconductor devices such as LEDs and laser diodesemploy quantum wells with uniform material compositions, such as thequantum wells described above in Table 1. Thus, conventional quantumwell designs employ very uniform material and require the crystal growerto go to great lengths to ensure that the quantum well material is ashomogeneous as possible.

While such active layers are well suited for many applications, they maynot be optimal for some material systems, including those necessary foraccessing the green and deep UltraViolet (UV) wavelength regions. Thegreen wavelength region may be considered to be from about 470nanometers (nm) to about 550 nm, while the deep UV wavelength region maybe considered to be from about 200 nm to about 365 nm. For example, withIn_(x)Ga_(1-x)N, the high In component needed to attain a bandgap forgreen emission usually leads to uncontrollable segregation of InN or GaNduring material growth. For the deep UV wavelength range, it becomesincreasingly difficult to grow high quality quantum wells with the highAl-containing AlGaN. Example embodiments address these and otherdisadvantages of the conventional art.

FIG. 1 is a graph 100 illustrating the L-I (light vs. current)characteristics of 300 μm×300 μm deep UV LEDs that employs conventionalquantum well active layers having uniform material composition. FIG. 2is a graph 200 illustrating the L-I characteristics of 200 μm×200 μmdeep UV LEDs that employ conventional quantum well active layers havinguniform material composition. Graphs 100 and 200 are referenced in thedescription of the example embodiments below for comparison purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the L-I characteristics of 300 μm×300 μmdeep UV LEDs that employ conventional quantum well active layers havinguniform material composition.

FIG. 2 is a graph illustrating the L-I characteristics of 200 μm×200 μmdeep UV LEDs that employ conventional quantum well active layers havinguniform material composition.

FIG. 3 is a graph illustrating the L-I characteristics of 300 μm×300 μmdeep UV LEDs that employ superlattice quantum well active layers inaccordance with example embodiments.

FIG. 4 is a graph illustrating the L-I characteristics of a 200 μm×200μm deep UV LEDs that employ superlattice quantum well active layers inaccordance with example embodiments.

FIG. 5 is a graph that compares the V-I (voltage-current)characteristics of one of the deep UV LEDs characterized in FIG. 3 tothe V-I characteristics of one of the deep UV LEDs that is characterizedin FIG. 1.

FIG. 6 is a graph that illustrates the spectrum of a LED that employssuperlattice quantum wells with Si-doped high bandgap sections inaccordance with example embodiments.

FIG. 7 is a cross-sectional diagram illustrating an embodiment of animhogeneous superlattice structure.

FIG. 8 is a flowchart illustrating a process included in example methodembodiments for fabricating a light-emitting device.

FIG. 9 is a flowchart illustrating an example sub-process included inthe process of FIG. 8.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In one form of the presently described embodiments, an inhomogeneousquantum well design is disclosed whereby the material composition of thequantum well is controllably altered during material growth. Accordingto some embodiments, during MOCVD, the concentration of one or moremetal organic source gases is modulated during crystal growth. Accordingto other embodiments, other crystal growth techniques such as MolecularBeam Epitaxy (MBE) or Chemical Beam Epitaxy (CBE) may be used wherebyone or more parameters such as temperature, pressure, or gas flow ratesare varied by design during crystal growth.

While embodiments may employ different inhomogeneous quantum welldesigns, in some embodiments the composition fluctuation of the quantumwell is made periodic. For purposes of this disclosure, such a quantumwell design will be referred to as a short period superlattice quantumwell, or superlattice. The superlattice design allows deposition of thinalternating layers of different materials to achieve an average overallalloy composition, making it useful in cases where direct growth of adesired material alloy is difficult. For example, alternating layers ofthin binary InN and GaN may be deposited to achieve an InGaN regionhaving a desired average In content.

In some embodiments, the superlattice design may be employed for eachquantum well within a multiple quantum well active layer. According tosome embodiments, certain sections of the superlattice can also be dopedwith impurities to lower device resistance or to counter built-inpolarization fields. Other advantages of example embodiments will beapparent in the description that follows.

One particular example of an inhomogeneous active region in InGaN is asuperlattice design where the In concentration is controllably alteredfrom layer to layer. Within such an active region the wave function forthe electrons and holes will have more weight on the In-rich regions andrelatively less on the Ga-rich regions. The effective band gap willtherefore be lower in such an inhomogeneous active region in comparisonto a homogeneous active region that has the same average In and Gacompositions. The lattice mismatch is largely determined by the averagecompositions in the quantum well region and the barrier region. Thebarrier region can be grown with a homogeneous In(Ga) composition.Therefore, by forming an inhomogeneous quantum well region, and ahomogeneous barrier region, one can achieve a larger band offset withthe same lattice mismatch than is possible if the quantum well region ishomogeneous. This will improve the performance of the emitter.

A specific example is a green emitter with an active region having anaverage In composition of 30% and a barrier region with an average Incomposition of 15%. If both barrier and active regions had homogeneousIn compositions then the difference in energy band gaps would be about0.5 eV. However, if the active region is grown with a variable Incomposition, then the difference in band gaps is larger because of aredshift in the inhomogeneous active region, but the lattice mismatchwill be unchanged. In this context, “redshift” refers to theinhomogeneous active layer having an effective bandgap that is smallerthan the bandgap of a similar active layer that has the same averagecomposition as that of the inhomogeneous active layer. This redshift, orlower effective bandgap in the active layer, allows a larger effectiveband offset (difference in bandgaps) between the active layer and thebarrier than what can be attained from a homogenous active layer.

According to example embodiments, the growth of the active region may beperformed by varying the In and Ga flow rates during MOCVD. In otherexample embodiments, the growth of the active region may be performed byalteration of the temperatures of the elemental In and Ga sources duringMBE.

To demonstrate the concepts described above, the inventors have grown,fabricated, and characterized deep UV LEDs operating at the 345 nmwavelength range. Devices utilizing conventional quantum wells withhomogeneous active regions, such as the LED of Table 1, were comparedwith those where the active regions were replaced with inhomogeneoussuperlattice structures in accordance with example embodiments.

In one superlattice quantum well design according to an exampleembodiment, the conventional growth sequence for the quantum well layersshown in Table 2 was modified by breaking up the original 120 secondgrowth time into five 20-second periods plus two 10-second periods. Thegas flow rates for MOCVD are controlled during the five 20-secondperiods such that a fluctuation of the gas flow rates is achieved over atotal of 100 seconds. One of the 10-second periods immediately precedesthe five 20-second periods, while the other one of the 10-second periodsimmediately follows the five 20-second periods. During the 10-secondperiods, the gas flow remains constant. Table 3, which is presentedbelow, illustrates the MOCVD gas flow conditions for the examplesuperlattice design described above.

TABLE 3 Time (sec) TMG (cc/min) TMA (cc/min) TMI (cc/min) Repetition 0-10 0.9 0.4 80 10-20 1.8 0 80 4 20-30 0 0.8 80 110-120 0.9 0.4 80

Table 3 illustrates a five-period superlattice, because the 20 secondperiod between 10 and 30 seconds is repeated four additional times. Thetotal number of seconds for the gas flow conditions of Table 3 is thesame as for the conventional gas flow conditions illustrated in Table 2.During the first and last ten seconds of Table 3, the gas flowconditions are the same as Table 2. The superlattice structure isestablished over the 100 seconds between 10 and 110 seconds.

As illustrated in Table 3, during each 20 second period when thesuperlattice structure is established, the TMG flows at 1.8 cc/min forthe first ten seconds, and then at zero cc/min for the next ten seconds.On the other hand, the TMA flow is zero cc/min for the first tenseconds, but 0.8 cc/min for the next ten seconds. The TMI remains at aconstant flow throughout the 120 second process. The section with 1.8cc/min TMG and 0 cc/min TMA would result in a gallium-rich film relativeto the film corresponding to the flow of 0.9 cc/min TMG and 0.4 cc/minTMA. Likewise, the section with 0 cc/min TMG and 0.8 cc/min TMA wouldresult in an aluminum-rich relative to the film corresponding to theflow of 0.9 cc/min TMG and 0.4 cc/min TMA.

Note that the average flow rates for the TMG and the TMA during seconds10-110 is the same as for the conventional flow rates illustrated inTable 2. Thus, the average composition of the five-period superlatticewould correspond to the composition of the average flows, namely, 0.9cc/min TMG and 0.4 cc/min TMA. A crystal grower could replace the MOCVDflow conditions of Table 2 with the flow conditions of Table 3 andachieve an inhomogeneous quantum well structure that has the sameaverage composition as the quantum well layers of Table 1 (layers 49 and53). Table 4, which is presented below, illustrates the epitaxialstructure of the inhomogeneous quantum well that is obtained using thegas flow rates described in Table 3.

TABLE 4 Layer Composition Thickness (nm) Repetition Comments 1In_(.01)Al_(.14)Ga_(.85)N .4375 Inhomogeneous 2 In_(.01)Ga_(.99)N .875 4quantum well, 3 In_(.01)Al_(.99)N 5.25 nm total 12In_(.01)Al_(.14)Ga_(.85)N .4375

FIG. 7 is a cross-sectional diagram illustrating an example embodimentof an inhomogeneous superlattice structure. In particular, FIG. 7illustrates all twelve layers of an inhomogeneous quantum well 700 thatis described above in Table 4. FIG. 7 is drawn for illustrative purposesand is not drawn to scale.

Referring to FIG. 7, the cross-hatched layers 705, 760 of the quantumwell 700 correspond to layers 1 and 12 in Table 4. The clear layers 710,720, 730, 740, 750 correspond to layers 2, 4, 6, 8, 10, respectively, ofTable 4. The dotted layers 715, 725, 735, 745, 755 of the quantum well700 correspond to layers 3, 5, 7, 9, 11, respectively, of Table 4. Thefive-period superlattice structure consisting of layers 710, 715, 720,725, 730, 735, 740, 745, 750, and 755 is clearly visible in FIG. 7.

Tables 3, 4 and FIG. 7 illustrate a further advantage of exampleembodiments. The conventional homogeneous quantum well layers (layers 49and 53) of Table 1 are composed of a relatively thick quaternary alloy,that is, the homogeneous quantum well layer has four principalconstituents. According to example embodiments, the conventional quantumwell layers can be replaced using an inhomogeneous superlatticestructure like the one illustrated in FIG. 7. As shown by layers 2 and 3of Table 4, the example superlattice structure consists of alternatinglayers of relatively thin ternary alloys, that is, a layer having threeprincipal constituents. While the inhomogeneous superlattice structurehas the same average composition as the conventional homogeneous quantumwell layer, it is easier to control the growth of multiple layers ofternary alloys that are relatively thin rather than the growth of asingle layer of a quaternary alloy that is relatively thick.

Similarly, according to other example embodiments a conventionalhomogenous quantum well layer consisting of a ternary alloy could bereplaced by an inhomogenous superlattice structure consisting ofalternating layers of binary alloys, or alloys having two principalconstituents. For example, the ternary alloy InGaN has a tendency touncontrollably segregate into random clusters of InN or GaN as theconcentration of indium increases, which is very undesirable. Byreplacing a single InGaN layer with multiple alternating layers of InNand GaN that can be precisely controlled, the same average compositioncan be achieved without the danger of segregation that is inherent inthe single homogeneous layer.

Table 5, which is presented below, illustrates the MOCVD gas flowconditions for another five-period superlattice quantum well designaccording to an example embodiment where only the TMG flow is variedwithin each period of the superlattice. In this design, the section with10.2 cc/min TMG and 0.4 cc/min TMA would result in a gallium-rich filmrelative to the film corresponding to the flow of 0.9 cc/min TMG and 0.4cc/min TMA. Likewise, the section with 0.6 cc/min TMG and 0.4 cc/min TMAwould result in an aluminum-rich film relative to the film correspond tothe flow of 0.9 cc/min TMG and 0.4 cc/min TMA.

TABLE 5 Time (sec) TMG (cc/min) TMA (cc/min) TMI (cc/min) Repetition 0-10 0.9 0.4 80 10-20 1.2 0.4 80 4 20-30 0.6 0.4 80 110-120 0.9 0.4 80

The average composition for the five-period superlattice quantum wellstructure resulting from the gas flow rates of Table 5 remains the sameas the average composition for the five-period superlattice quantum wellstructure resulting from the gas flow rates of Table 3. Table 6, whichis presented below, illustrates the epitaxial structure of theinhomogeneous superlattice that is obtained using the gas flow ratesdescribed in Table 5.

TABLE 6 Layer Composition Thickness (nm) Repetition Comments 1In_(.01)Al_(.14)Ga_(.85)N .4375 Inhomogeneous 2In_(.01)Al_(.11)Ga_(.88)N .875 4 quantum well, 3In_(.01)Al_(.17)Ga_(.82)N 5.25 nm total 12 In_(.01)Al_(.14)Ga_(.85)N.4375

FIG. 3 is a graph 300 illustrating the L-I characteristics of 300 μm×300μm deep UV LEDs that employ superlattice quantum well active layers inaccordance with example embodiments, while FIG. 4 is a graph 400illustrating the L-I characteristics of 200 μm×200 μm deep UV LEDs thatemploy superlattice quantum well active layers in accordance withexample embodiments. The LEDs characterized in FIGS. 3 and 4 werefabricated on the same wafer using the superlattice quantum well designillustrated in Tables 5 and 6.

For comparison with FIGS. 3 and 4, FIG. 1 is a graph 100 illustratingthe L-I characteristics of 300 μm×300 μm deep UV LEDs that employconventional quantum well active layers having uniform materialcomposition, while FIG. 2 is a graph 200 illustrating the L-Icharacteristics of 200 μm×200 μm deep UV LEDs that employ conventionalquantum well active layers having uniform material composition. Otherthan the quantum well layers, the epitaxial layers of the LEDscharacterized in FIGS. 3 and 4 were identical to the epitaxial layers ofthe LEDs characterized in FIGS. 1 and 2.

Referring to FIGS. 1, 2, 3, and 4, it can be seen that the peak lightoutput for the LEDs that employ the inhomogeneous superlattice quantumwell active layers according to example embodiments is roughly 0.7 mWgreater than the LEDs that employ the conventional quantum well activelayers having a uniform material composition. The L-I characteristicsshown in FIGS. 3 and 4 are excellent results for λ=343 nm.

FIG. 5 is a graph 500 that compares the V-I characteristics of one ofthe deep UV LEDs characterized in FIG. 3 to the V-I characteristics ofone of the deep UV LEDs that is characterized in FIG. 1. FIG. 5illustrates that for a given current, less voltage is needed for thesuperlattice design than for the conventional design. Evidently, thesuperlattice quantum well device according to example embodimentsperforms better than the conventional quantum well device in terms ofboth power output and lower voltages.

A special case of the superlattice multiple quantum well is thesuperlattice single quantum well, where a simple single-periodsuperlattice encompasses the entire active region. Another exampleembodiment would be a superlattice multiple quantum well active regionwhere the barrier layers are constructed of superlattices. The quantumwells within this active region can be conventional quantum wells withhomogeneous composition, or they can also be superlattices as alreadydescribed.

Other inhomogeneous quantum well designs can be employed. For example,variable-period-superlattice quantum wells would be a straightforwardextension of the devices described above. In such a design, theperiodicity of material fluctuation changes across the quantum well.Additionally, although the discussion thus far has focused on MOCVDgrowth, MBE may be a growth technique of choice if low-temperatureconditions such as those for high indium incorporation are desired.

In Table 3, the gas source flow rates for TMG and TMA are simultaneouslyswitched during superlattice transitions, but this simultaneoustransition is not essential. One gas source can be switched first fromone flow rate to another before a subsequent flow rate change is made onthe second or third gas source. The transition delays between gassources would produce a smoother, more gradual transition in materialcomposition fluctuations. These transition designs would also apply toembodiments where the material composition changes are affected bychanging growth parameters other than source gas flow rates. In the caseof MBE or CBE, for example, the growth parameter of choice may betemperature.

It is common to grow nitride-based light emitting devices on c-planesubstrates. Unfortunately, this crystal orientation leads topolarization field at the quantum well layers. The built-in electricfield causes energy band tilting that hampers carrier recombination. Tohelp counteract the polarization field, Si impurity doping has been usedat the barriers in multiple quantum well structures. An extra degree ofdesign flexibility could be achieved by doping the quantum wells withoutseriously impairing the recombination process. According to exampleembodiments, a possible technique would be to use superlattice quantumwell active layers and to dope only the high bandgap sections of thesuperlattice. The inventors have demonstrated just such a device havinggood device performance.

FIG. 6 is a graph 600 that illustrates the spectrum of a LED thatemploys superlattice quantum wells with Si-doped high bandgap sectionsin accordance with example embodiments. The superlattice structure ofthe LED characterized in FIG. 6 was obtained using Table 5, with theexception that Si doping was added during the 0.6 cc/min TMG, 0.4 cc/minTMA growth segment for the superlattice. While better-performing deviceshave been obtained from undoped quantum wells, the result suggests thatdoped superlattice quantum wells are also a viable alternative.

FIG. 8 is a flowchart illustrating a process 810 included in a methodembodiment 800 for fabricating a light-emitting device. Referring toFIG. 8, the process 810 includes modulating a crystal growth parameterto grow a quantum well layer that is inhomogeneous and that has anon-random composition fluctuation across the quantum well layer. Thoseof ordinary skill in the art will recognize that process 810 may involvea multitude of repeating sub-processes. For example, for the quantumwell 700 having the five-period superlattice structure illustrated inFIG. 7, the same epitaxial layer deposition process is performed fivetimes to form the layers 710, 720, 730, 740, and 750. Likewise, adifferent epitaxial layer deposition process is performed five times todeposit the layers 715, 725, 735, 745, and 755. Any known technique forforming or depositing an epitaxial layer may be used. For example,MOCVD, MBE, or CBE may be used in different embodiments.

FIG. 9 is a flowchart illustrating an example sub-process 900 that maybe included in the process 810 of FIG. 8. Referring to FIG. 9 thesub-process 900 includes modulating the crystal growth parameter suchthat the composition fluctuation is substantially periodic. For example,Table 3 illustrates that the TMA and TMG source gases in a MOCVD processare periodically turned on and off over a time span of 100 seconds.Those of ordinary skill will recognize and appreciate that the periodicmaterial composition fluctuation of the epitaxial layers that resultfrom the process illustrated in Table 3 does not exhibit a perfectlyon/off characteristic (analogous to a step function in an electricalsignal), but that the presence of some residual source gases in theprocess chamber will tend to make the transition between layers ofdifferent composition a more gradual one.

It will be appreciated that variations of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A method of fabricating a light emitting device comprising: growing acrystal layer as a quantum well layer within the light emitting device;and modulating a process parameter during the growing of the quantumwell layer such that the quantum well layer is inhomogeneous and has anon-random, periodic, composition fluctuation across the quantum welllayer; and doping a section of the quantum well layer having a higherbandgap than other sections of the quantum well layer.
 2. The method ofclaim 1, wherein modulating the process parameter comprises alternatelyincreasing and decreasing a flow rate of a metal organic source gasduring a metal organic chemical vapor deposition process.
 3. The methodof claim 2, wherein alternately increasing and decreasing the flow rateof the metal organic source gas comprises alternately starting andstopping the flow rate.
 4. The method of claim 1, wherein modulating theprocess parameter comprises alternately increasing and decreasing aprocess temperature during a molecular beam epitaxy process.
 5. Themethod of claim 1, wherein modulating the process parameter comprisesalternately increasing and decreasing a process pressure during achemical beam epitaxy process.